Fast atom bombardment (FAB) has become one of the most widely utilized techniques for obtaining mass spectra of thermally labile and/or very polar substances. In conjunction with tandem MS (MS- MS), the fragmentation information so vital for structure elucidation can be obtained from ions normally not observed to fragment and from mixtures where the parent ion-daughter ion relationships are not known. This project involves instrumental developments in MS-MS and application of FAB/MS and FAB/MS-MS for structure elucidation. In the area of instrumental developments we are currently investigating the effect of energy loss on the MS-MS data obtained by a linked scan of MS-II and for how these effects can best be compensated. Applications of FAB/MS and FAB/MS/MS for structure elucidation include: the structure elucidation of phospholipids containing unusual fatty acid and headgroups; peptides in which an amide linkage has been modified; and glutathione, cysteine and N- acetylcysteine conjugates of xenobiotics, both synthetic and conjugates isolated from challenged animals.